Source: http://www.zoharbabin.com/how-to-do-noise-reduction-using-ffmpeg-and-sox/ #videos

How to reduce noise on videos using linux.

Need a quick, easy and free way of cleaning up a video’s audio from background noise ? Follow the steps below to remove background audio from videos.

First get and install the tools:

FFMPEG.
SoX – Sound eXchange.
Then clean your noise!

Split the audio and video streams into 2 seperate files:
The VIDEO stream: ffmpeg -i input.mp4 -vcodec copy -an tmpvid.mp4
The AUDIO stream: ffmpeg -i input.mp4 -acodec pcm_s16le -ar 128k -vn tmpaud.wav 
Generate a sample of noise from the audio of the file:
ffmpeg -i input.mp4 -acodec pcm_s16le -ar 128k -vn -ss 00:00:00.0 -t 00:00:00.5 noiseaud.wav
-ss: the time offset from beginning. (h:m:s.ms).
-t duration: the duration (h:m:s.ms) of audio segment to cut that represents the noise.
Choose a segment of the audio where there’s no speech, only noise (e.g. speaker was silent for a sec).
 Generate a noise profile in sox:
sox noiseaud.wav -n noiseprof noise.prof
Clean the noise samples from the audio stream:
sox tmpaud.wav tmpaud-clean.wav noisered noise.prof 0.21
Change 0.21 to adjust the level of sensitivity in the sampling rates (I found 0.2-0.3 often provides best result).
Merge the audio and video streams back together:
ffmpeg -i tmpvid.mp4 -i tmpaud-clean.wav -map 0:v -map 1:a -c:v copy -c:a aac -b:a 128k out.mp4
I’m thinking of automating this via a web interface that allows uploading a video file, selection of the noise sample and playing with the noisered levels producing a cleaned version… think it’s a useful tool?
Thanks for all the contributors in the comments!